In recent years, with increased concern over environmental issues, there is more and more need for accurate and cost effective measurement devices and methods for the detection of gases in the atmosphere. There is a demand both for measurements of pollutant gases at ground level, due to their direct effect on human health, and also within the atmosphere for atmospheric analysis and research.
Ozone, nitrogen dioxide and nitrogen monoxide are gases of particular relevance and it is desirable to monitor concentrations of these gases in air. Particular applications include roadside monitoring and atmosphere content monitoring.
The paper “Indirect Determination of Nitrogen Oxides by Chemiluminescence Techniques” (R. Guicherit, Atmos Environ. 1972 November; 6(11) p. 807-14)REFERENCE, 1972) shows a method and apparatus for determining nitrogen dioxide in air samples. This process involves use of a chemiluminescent detection system for ozone, which is produced by means of the effect of ultraviolet light on the equilibrium reaction:NO2+O2⇄NO+O3
Ultraviolet light pushes the equilibrium to the right, forming nitrogen monoxide and ozone. This process involves irradiating a gas sample with ultraviolet light and then determining the final ozone concentration.
The resulting system is complex and the authors found it essential to pass incoming gas through scrubbers to remove ozone and to oxidise nitrogen monoxide to nitrogen dioxide before beginning irradiation.
This system further proposed that, once nitrogen dioxide had been determined, the concentration of nitrogen monoxide could subsequently be calculated after a separate reading of ozone produced by ultraviolet irradiation of a separate gas sample, scrubbed as before, with the addition of a further scrubber to remove ozone.
This process is the basis of a number of gas monitoring devices on sale at the present time. However, these devices are expensive (in the order of £10,000), complex and suffer from limitation in stability with time and in response to temperature changes.
Another known technique for determining total nitrogen oxide content in air is described in U.S. Pat. No. 3,973,914 to van Heusden. This functions by converting nitrogen dioxide to nitrogen monoxide which is then determined by monitoring chemiluminescence of the reaction between nitrogen monoxide and excess ozone. This functions by converting nitrogen dioxide to nitrogen monoxide which is then determined by monitoring chemiluminescence of the reaction between nitrogen monoxide and excess ozone.
This process is the basis of a number of gas monitoring devices on sale at the present time. However, these devices are expensive (in the order of £10,000), complex and suffer from limitations, particularly relating to the quantitative conversion of nitrogen dioxide to nitrogen monoxide.
A first aim of the present invention is to provide a cheaper, simpler and more reliable gas monitoring device, capable of accurate determination of nitrogen dioxide, nitrogen monoxide and/or ozone in air.
One use for a cheaper, simple and more reliable gas monitoring device would be for measuring atmospheric composition at high altitudes. Typically, this is carried out by attaching sensors to a helium filled weather balloon and to launch the balloon into the atmosphere. The sensor then transmits data, usually via a radio link, as the balloon passes up through the atmosphere. The fact that the sensor is attached to a balloon places severe restrictions in terms of weight and cost on the sensor design. The cost aspect can be particularly critical, because generally such sensors have to be treated as disposable, given that they may be difficult to locate once they fall back to the ground. All of this can make it extremely difficult to produce a sensor which provides appropriately accurate readings that are continuous throughout the ascent of the balloon and which is capable of resisting the effects of high altitude and low temperature, where contaminants such as rain ice can affect sensor output.
Therefore, a second aim of the present invention is to provide a device that is suitably reliable, cheap and light to benefit high altitude gas monitoring.
Furthermore, high altitude gas measurement systems usually use a pump to draw air into a sensing arrangement. The pumps are expensive, relatively heavy and require considerable power to operate, increasing overall device weight.
Furthermore, with such sensors it can often be difficult to provide accurate readings because it is difficult to determine exactly the rate at which atmospheric air is flowing over the sensor and a further aim of the present invention is to provide sensing apparatus which mitigates this problem.